Analysis of limitations to CO₂ assimilation on exposure of leaves of two Brassica napus cultivars to UV‐B

Abstract

Apex and Bristol cultivars of oilseed rape {Brassica napus) were irradiated with 0-63 W m~^ of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bisphosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO2-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO2 assimilation.Key-words: Brassica napus; electron transport; fluorescence; leaf gas exchange; photosynthesis; photosystem II; Rubisco; ultraviolet-B radiation.Abbreviations: A, net CO2 assimilation rate; A^.^^, light-saturated net CO2 assimilation rate; Cj, intercellular CO2 concentration; Fo, F^, E^, minimal, maximal and variable fluorescence yields; F,^', EJ, E^, maximal, variable and steady-state fluorescence yields in a light-adapted state; ^max.RuBP' maximum potential rate of electron transport contributing to RuBP regeneration; imax.psii' rate of PSII electron transport at saturating light and CO2; PPFD, photosynthetically active photon flux density; RuBP, ribulose 1,5-bisphosphate; Rubisco, ribulose 1,5-bisphosphate carboxylase/oxygenase; V^..^^^, maximum carboxylation velocity of Rubisco; ^psn, relative quantum efficiency of PSII photochemistry.

The relationship between CO2 assimilation, photosynthetic electron transport and water–water cycle in chill‐exposed cucumber leaves under low light and subsequent recovery

“…In comparison, the chill-induced reduction in photosynthetic capacity was accompanied by a decrease in both V cmax and J max . Inactivation or loss of Rubisco would reduce the carboxylation efficiency (V cmax ) while the reduction in J max is associated with a diminution of sedoheptulose-1,7-bisphosphatase, a key regulatory enzyme in the Calvin cycle (Allen et al 1997(Allen et al , 2000Nogués & Baker 2000;Ölcer, Lloyd & Raines 2001). Our results clearly showed that chill for 10 d had little effect on Rubisco content, total Rubisco activity but significantly decreased the initial Rubisco activity (Table 1).…”

Antioxidant defence in UV-irradiated tobacco leaves is centred on hydrogen-peroxide neutralization

“…Samples were pooled for SOD and APX activities in gels and these were measured twice, representing the two biological repetitions. Student's t-test was used to compare means and to calculate P-values, and differences were considered significant at P < 0.05. to have a direct effect on stomata (Nogues et al, 1999), in addition to changes in mesophyll photosynthesis including a decrease in both the amount and the activity of Rubisco (Strid et al, 1990;Allen et al, 1997). Supplemental UV did not result in a significant change in stomata conductance and caused a larger decrease in photosynthetic CO 2 uptake than in photochemical yield, suggesting that a partial inactivation of dark reactions contributes to the lower photosynthesis in UV treated leaves.…”

Growth and physiological responses of sunflower plants exposed to ultraviolet-B radiation

“…Thus, in sunflower plants other factors may be limiting CO 2 assimilation. ALLEN et al (1997) concluded that the decrease in the rate of CO2 assimilation of oilseed rape plants under enhanced UV-B radiation is a consequence of loss in Rubisco activity and content.…”